Phytophthora infestans is a specialized pathogen of Solanaceae (the tomato family) that causes late blight of both tomatoes and potatoes. Until recently, P. infestans only one mating strain (A1) had spread throughout the world. However, in the 1950s, a second strain of P. infestans, A2, was discovered on tomatoes in the highlands of central Mexico.  This discovery sparked a renewed awareness of the deadly pathogen. Problems first arose in 1978 when the A2 strain, thought to exist solely in Mexico, appeared in Switzerland and began mating with the A1 strain.  The exact method of the A2 strain migrating to Europe is not known, but it is almost certainly due to human activities. Now that A2 has spread to Europe, America, and Asia, farmers are having difficulty protecting their crops from the deadly pathogen.  The new presence of A2 has allowed P. infestans to mate and produce recombinant genotypes. Recombination between A1 and A2 has produced resistance to widely used fungicides and pesticides such as Bordeaux mixture, metalaxyl, and acyalanine.  Even after spending $3 billion annually worldwide to combat P. infestans, in 1990, some farmers were forced to burn their entire crops as their harvest was infected with an exotic strain resistant to the pesticide metalaxyl. Since P. infestans has been increasingly more difficult to control, countries worldwide are focusing on selecting tomato and potato cultivars that exhibit high and durable resistance to the pathogen. Scientists have found ways to incorporate late blight resistance from wild Mexican potatoes into common American spuds. This new technique, known as embryo rescue, involves removing healthy embryos from the infected plant seed and growing these embryos on a culture medium. A hybrid from this embryo rescue is then crossed with cultivated potatoes – resulting in offspring that are resistant to P. infestans. With this new technique and the heightened awareness of P. infestans, the pathogen is hopefully under control. However, if left to run rampant, P. infestans might completely destroy the world’s beloved tomato and potato crops.  --J. Triola